Log in with your Google account and select which calendars you want to synchronize than click Save.

On your Windows Phone device, go to settings > email + accounts

Locate your Google account, long tap and select Sync

Once the sync completes you should have all the desired calendars visible in the main Calendar app.

Note that the instruction above apply for devices synchronizing via CalDAV. If you are a Google Apps for business user and have your account configured to synchronize via EAS you can still control which caledars to synchronize by visiting the following page from the web browser on your Windows Phone device: https://m.google.com/sync/settings/